The effects of specific DNA lesions on cellular metabolism are poorly understood. Double-strand breaks (DSBs) is one category of lesions that is of considerable concern. They can occur as primary events as a consequence of repair. In addition they may appear during normal DNA metabolic events involving recombination. DSBs are known to be inducers of recombination. In order to understand the consequences of DSBs on cellular DNA and their role as inducers of recombination, a system has been designed in the yeast Saccharomyces cerevisiae to study the effect of defined DNA double-strand breaks in plasmids on recombination and repair at various chromosomal sites. In this system, a diploid yeast strain with a heteroallelic marker on homologous chromosomes harbors two plasmids: a low copy CEN plasmid that carries a gene coding for the site-specific endonuclease HO under the control of the inducing promoter GAL, and a high copy 2 Mu plasmid that carries a site at which the HO endonuclease cuts. Since the chromosomal HO-cut sites are deleted in this strain, depression of HO should generate a defined multiple DSB Only at HO-cut sites carried by the 2 Mu plasmids. Preliminary experiments indicate the depression of the HO gene and subsequent DSBs in plasmids causes induction of recombination in the chromosome. The mechanism of this trans- acting effect of DSBs in being investigated in terms of effects on survival, mutation, and recombination and genetic control.